Energy, water, and electrolyte balance are among the most closely regulated parameters in vertebrates. Obligate loss occurs continuously and uncorrected deficits become life threatening. Only one process, ingestion of foods or fluids, can replenish supplies of nutrients, electrolytes, and water. The goal of this research proposal is to understand the neural mechanisms that govern the decision to ingest or reject the contents of the oral cavity. The neural substrates for this decision are complete in the brainstem. The oral tactile, thermal, and gustatory afferent activity that guides ingestive behavior first reaches the brain in the pons and medulla. The response elicited by these oral stimuli can be switched from ingestion to rejection by vagal visceral activity that first reaches the CNS in the medulla. The motor neurons that produce ingestive behavior also are located in the brainstem. Finally, precollicular decerebrate preparations, which utilize only the brainstem to integrate behavior, exhibit normal ingestion and rejection behavior and, given an appropriate visceral stimulus, can switch their response from ingestion to rejection. Nevertheless, learned control of ingestion requires connections to the forebrain. Using behavioral, neuroanatomical, neurochemical, and electrophysiological analysis, this project will test hypotheses about the functions of the nucleus of the solitary tract (NST) and the parabrachial nuclei (PBN) in processing taste and visceral afferent activity relevant to ingestion. Specific experiments examine (1) how the gustatory senscry relays and the ventral forebrain interact to control of taste-guided behaviors, (2) the influence of visceral and forebrain neural activity on PBN gustatory neurons, and (3) the role of brainstem sensory nuclei in processing gustatory and oral trigeminal neural activity. In addition to contributing toward the basic understanding of how the nervous system coordinates diverse sensory information into precise physiological and behavioral controls, this research has direct relevance to the sensory mechanisms involved in the etiology of a variety of morbid conditions. Specifically, excess ingestion of carbohydrates, fats, and sodium is highly correlated with, and may be causally related to obesity, diabetes, and hypertension.